Tissue is used as a component source in several kinds of medical devices. In some of these applications, it may be desirable to modify the thickness and/or surface characteristics of tissue by removing all or a portion of one or more layers of the tissue. Often, unwanted tissue is removed and discarded while the remainder is incorporated into the device. Percutaneous heart valves are an exemplary application that involves thinning down of tissue before the tissue is incorporated into a heart valve.
As background, there are four valves in the heart that serve to direct blood flow through the two sides of the heart. On the left (systemic) side of the heart are: (1) the mitral valve, located between the left atrium and the left ventricle, and (2) the aortic valve, located between the left ventricle and the aorta. These two valves direct oxygenated blood from the lungs through the left side of the heart and into the aorta for distribution to the body. On the right (pulmonary) side of the heart are: (1) the tricuspid valve, located between the right atrium and the right ventricle, and (2) the pulmonary valve, located between the right ventricle and the pulmonary artery. These two valves direct de-oxygenated blood from the body through the right side of the heart and into the pulmonary artery for distribution to the lungs, where the blood becomes re-oxygenated in order to begin the circuit anew.
All four of these heart valves are passive structures in that they do not themselves expend any energy and do not perform any active contractile function. They consist of moveable “leaflets” that open and close in response to differential pressures on either side of the valve. The mitral and tricuspid valves are referred to as “atrioventricular valves” because they are situated between an atrium and ventricle on each side of the heart. The mitral valve has two leaflets and the tricuspid valve has three. The aortic and pulmonary valves are referred to as “semilunar valves” because of the unique appearance of their leaflets, which are shaped somewhat like a half-moon and are more aptly termed “cusps”. The aortic and pulmonary valves each have three cusps.
Heart valves may exhibit abnormal anatomy and function as a result of congenital or acquired valve disease. Congenital valve abnormalities may be well-tolerated for many years only to develop into a life-threatening problem in an elderly patient, or may be so severe that emergency surgery is required within the first few hours of life. Acquired valve disease may result from causes such as rheumatic fever, degenerative disorders of the valve tissue, bacterial or fungal infections, and trauma.
Since heart valves are passive structures that simply open and close in response to differential pressures on either side of the particular valve, the problems that can develop with valves can be classified into two categories: (1) stenosis, in which a valve does not open properly, and (2) insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload placed on the heart. The severity of this increased stress on the heart and the patient, and the heart's ability to adapt to it, determine whether the abnormal valve will have to be surgically replaced (or, in some cases, repaired).
Valve repair and valve replacement surgery is described and illustrated in numerous books and articles, and a number of options, including artificial mechanical valves and artificial tissue valves, are currently available. Prosthetic heart valves are described, for example, in U.S. Patent Publication No. 2004/0138742 A1.
Recently, there has been interest in minimally invasive and percutaneous replacement of cardiac valves. Percutaneous replacement of a heart valve does not involve actual physical removal of the diseased or injured heart valve. Rather, the defective or injured heart valve typically remains in position. The replacement valve typically is inserted into a balloon catheter and delivered percutaneously via the vascular system to the location of the failed heart valve.
In the context of percutaneous, pulmonary valve replacement, US Patent Application Publication Nos. 2003/0199971 A1 and 2003/0199963 A1, both filed by Tower, et al. describe a valved segment of bovine jugular vein, mounted within an expandable stent, for use as a replacement pulmonary valve. As described in the articles: “Percutaneous Insertion of the Pulmonary Valve”, Bonhoeffer, et al., Journal of the American College of Cardiology 2002; 39: 1664-1669 and “Transcatheter Replacement of a Bovine Valve in Pulmonary Position”, Bonhoeffer, et al., Circulation 2000; 102: 813-816, the replacement pulmonary valve may be implanted to replace native pulmonary valves or prosthetic pulmonary valves located in valved conduits. Other implantables and implant delivery devices also are disclosed in published U.S. Pat. Application No. 2003-0036791-A1 and European Patent Application No. 1 057 460-A1.
Assignee's co-pending U.S. Provisional Patent Application titled APPARATUS FOR TREATMENT OF CARDIAC VALVES AND METHOD OF ITS MANUFACTURE, in the names of Philippe Bonhoeffer and Debra Ann Taitague et al., filed Nov. 19, 2004, bearing, and assigned U.S. Ser. No. 60/629,468 (hereinafter referred to as the “Bonhoeffer and Taitague Application”) describes innovative, percutaneous heart valves for use as a replacement pulmonary valve. Like the valves described by Tower et al., the heart valves of this co-pending application incorporate a valved segment of bovine jugular vein, mounted within an expandable stent.
The tissue source for the percutaneous heart valves described in the Tower, Bonhoeffer, and Bonhoeffer and Taitague Application documents cited herein preferably is a valved segment of a bovine jugular vein. The bovine jugular vein has many properties making it suitable for use in a percutaneous heart valve. However, the size of the venous wall of this bovine tissue tends to be too thick to be used for percutaneous insertion. Fortunately, the wall thickness can be reduced significantly without interfering with the valve function. After removal of unnecessary tissue from the external venous wall, the modified tissue is sutured to a stent. The device is cross-linked with a buffered gluteraldehyde solution, sterilized and stored in an alcoholic gluteraldehyde solution according to industry protocols.
Conventionally, manual techniques are used to reduce the thickness profile of the venous wall. That is, an operator uses appropriate implements to remove unwanted tissue by hand. This process of thinning the venous wall is described in the Tower, Bonhoeffer, and Bonhoeffer and Taitague Application documents cited above. The manual process will produce an excellent product, but it nonetheless suffers from at least two drawbacks. First, the manual modification of the tissue is painstaking and laborious. It would be highly desirable to provide a tissue reduction methodology that can be carried out in less time. Second, the scrap rate of the manual technique can be quite high. It would be highly desirable to provide a tissue reduction methodology that modifies the thickness of the bovine venous wall with much higher yields.